bigcouch / cloudant support for couchexport

- should be non-breaking for couch as well
- convert seq property to a string
- add notifications when we try to export by seq (is deprecated)
- only do seq-based comparisons if we are on regular couch
- update tests
Merge pull request #33 from dimagi/cloudant

bigcouch / cloudant support for couchexport
Merge branch 'master' into celery-upgrade